Compared to a Neandertal's jutting mug, a modern human face is flat--tucked under its brain case in a vertical line. Now a researcher says this facial makeover stemmed from a single change in the development of the skull. The finding, reported in tomorrow's Nature, suggests that the robust Neandertals were a separate lineage from our own species, Homo sapiens.

Anthropologists have long argued over whether certain facial differences indicate that modern humans and Neandertals are separate species, or whether those traits are merely the result of differences in lifestyle, such as eating different foods or using teeth as tools. To get a new perspective, Rutgers University paleoanthropologist Daniel Lieberman used x-rays and computed tomography to analyze the skulls of 100 recent humans, 69 chimpanzees, and 17 extinct humans. He found that the central bone at the base of the brain case, called the sphenoid, is about 30% shorter in modern humans than in Neandertals or other ancient humans.

Since previous work has shown that the sphenoid influences the development of skull shape and the amount that the face projects, Lieberman concludes that shortening this single bone dramatically changed the faces of our ancestors, bringing them into a more vertical alignment and rounding their skulls. "This shortening of the sphenoid occurred once, probably in an African ancestor, and repositioned the face, tucking it under the brain case," says Lieberman. That this change is found neither in Neandertals nor in a more ancient human ancestor called H. heidelbergensis suggests a shortened sphenoid is a defining trait of our species. "It's a real, unique, derived feature found only in modern humans," says Lieberman.

Other paleoanthropologists are impressed with the work, which adds to a growing body of fossil and genetic evidence that humans and Neandertals are distinct species. "This is the first time we have a biologically realistic explanation of how a rounded human skull is put together," says Harvard University paleoanthropologist David Pilbeam. "And it seems to clearly separate modern humans from Neandertals and other archaics."